1. Field of the Invention
This invention relates generally to pumps and more particular to the metering-type pumps commonly used in analytical instruments.
2. Description of the Prior Art
Some analytical instruments have need for a metering-type pump to accurately measure quantities of solutions. For example, if an analytical instrument is to be used to perform a 1:100 dilution, a fixed displacement metering-type pump could be used to pump one part of a first solution into a vessel and then to pump one hundred parts of a second solution into the vessel. It may be appreciated, then, that a single 1:100 dilution would require 101 cycles of a fixed displacement metering-type pump, which not only is a very time-inefficient method of achieving such a dilution but which also subjects the pump to considerable wear.
A metering-type pump which has advantages over the above mentioned pump is one that is driven by a stepping motor. The discharge volume of the pump can be varied in a well-defined manner by selectively controlling the number of times the stepping motor is actuated. Such pumps are, however, very expensive.
Another way to overcome the disadvantages of the first mentioned dilution system is to construct an array of pumps of differing displacements and to couple them together in parallel. This again is an expensive solution to the problem and further suffers the disadvantage that a rinsing effect of the plurality of pumps is difficult, leading to a possible cross-contamination of the two solutions to be mixed.
In short, what the prior art fails to provide is a simple, inexpensive metering-type pump capable of quickly and efficiently performing such dilution operations and one which is designed to minimize the chance of cross-contaminating the solutions being handled.